1. Field of the Invention
This invention relates generally to the stabilization of fluid interfaces between a displaced fluid and a drive fluid. More specifically, a ferromagnetic fluid is employed as a driving fluid to control fingering effects at the interface between fluids of differing viscosities in the presence of a magnetic field containing a gradient of field intensity across the fluid interface.
2. Description of the Prior Art
Saffman and Taylor, Proc. Royal Soc. A., 245; 312-329 (1958), have shown that when a viscous fluid is driven through the voids of a porous medium by a less viscous fluid, the interface between the two fluids can be unstable for sufficiently high velocity. Rosensweig, Zahn and Vogler, "Thermomechanics of Magnetic Fluids", B. Berkovsky editor, McGraw Hill, 1978, pages 195-211, have analytically shown that if a layer of magnetizable fluid is used to push a more viscous fluid through a Hele-Shaw cell, the fluid interface can be stabilized using a uniform imposed magnetic field that is tangentially oriented to the interface. A Hele-Shaw cell allows flow between two parallel flat plates that are closely spaced. Tests in a Hele-Shaw cell confirmed the theoretical prediction and showed that viscous fingering can be prevented. Subsequently, Zahn and Rosensweig, IEEE Transactions on Magnetics, Vol. MAG-16, No. 2, March 1980, pp. 275-282 discuss a treatment of stability when magnetic field is uniform but obliquely oriented to the interface, in which case stabilization is less effective than for the tangential orientation.
In the field of oil recovery, fingering is a problem encountered in enhanced recovery processes including secondary and tertiary recovery techniques. Waterflooding is one of the common methods for secondary recovery. When dealing with relatively high viscosity crude oil, the process efficiency is very poor due to the tendency of the water or brine fluid to finger through and thereby bypass the more viscous oil. Moreover, much of the available oil is trapped in the formation by capillary and wettability forces. For this reason, both secondary and tertiary recovery processes may employ surfactants in the primary displacement fluid to lower the interfacial tension between water and oil to values less than about 1.0 dyne/cm.
The displacement fluid must be driven through a porous formation and if an aqueous solution is used as the driving fluid, fingering is usually observed due to differences in viscosities. In order to control fingering, it is customary to add a thickening agent, e.g., a polymer to increase the viscosity of the displacement or drive fluid and thereby minimize fingering. Polymers are subject to shear degradation and adsorption and retention in the porous medium leading to reduction of viscosity. Under other circumstances the use of polymers may lead to reduction in permeability or plugging.